The present invention relates to a semiconductor laser device, and more particularly to a semiconductor laser device in which a laser diode and a photodiode for monitoring an output of the laser diode are mounted on a mounting surface such as a stem. This type of semiconductor laser device is used, for example, as a component constructing a recording/reproducing device for CDs (compact discs), videodiscs, DVDs (Digital Versatile Discs) and so on.
Conventionally, a monitor submount type shown in FIGS. 6A and 6B are known as this type of semiconductor laser device (for example, JP 2001-267676A). FIG. 6A is a view of a laser diode seen from a direction perpendicular to a mounting surface thereof. FIG. 6B is a schematic cross section of the laser diode taken along line B—B of FIG. 6A (the same as in FIGS. 7A and 7B described below). The semiconductor laser device is constructed as follows. On a heat release block (heat sink) 115, there is mounted a rectangular parallelepiped Si (silicon) substrate 110 as a submount, in a surface of which a photodiode 104 is formed. Further, a laser diode 101 is mounted on a laser diode mounting electrode 109 formed on the surface of the silicon substrate 110. Reference numerals 105A, 105B and 105C respectively denote metal wires connected to a rear surface electrode (for simplicity, the wires are omitted in FIG. 6B). The laser diode 101 emits light forward (upward in FIGS. 6A and 6B) and backward (downward in FIGS. 6A and 6B). The laser light emitted forward (forward emitted light) is applied to an optical disc to be recorded/reproduced, while the laser light emitted backward (backward emitted light) is photoelectrically converted into a monitoring signal by the photodiode 104. The monitoring signal is used for controlling the output of the laser diode 101 by a control circuit not shown.
However, this semiconductor laser device has the submount composed of the Si substrate 110, where the thermal conductivity of Si is not so high (84 to 147 W/m·K) Therefore, the heat release characteristic (i.e. performance of releasing heat which the laser diode generates in operation) of the submount is not necessarily sufficient. There is a possibility that the laser diode 101 deteriorates in temperature characteristic or that the laser diode 101 is destructed due to thermal runaway.
Thus, another conventional semiconductor laser device is proposed as shown in FIGS. 7A and 7B (JP2001-345507A). This semiconductor laser device is constructed as follows. A rectangular parallelepiped Si (silicon) substrate 211, in a surface of which a photodiode 204 is formed, is mounted on a heat sink 215. In parallel to the substrate 211, there are mounted a separate rectangular parallelepiped submount 210 composed of an insulator such as AlN (aluminum nitride) and SiC (silicon carbide) having a large thermal conductivity. Further, a laser diode 201 is mounted on a laser diode mounting electrode 209A formed on a surface of the submount 210. The surface of the submount 210 is formed with another electrode 209B separated from the electrode 209A. Reference numerals 205A, 205B, 205C, 205D respectively denote metal wires (for simplicity, those are omitted in FIG. 7B). The submount 210 is improved in heat release characteristic (performance of releasing heat generated by the laser diode in operation) because the submount 210 is composed of a material having a large thermal conductivity such as AlN and SiC.
However, since the submount 210 on which the laser diode 201 is mounted and the Si substrate 211 formed with the photodiode 204 are separate from each other in this semiconductor laser device, the number of components increases, resulting in high cost.
Also, in order to stabilize monitoring signals outputted by the photodiode 204, the submount and the Si substrate must be accurately positioned in an assembling process and therefore manufacture is difficult.
Further, in order to make backward emitted light of the laser diode 201 be sufficiently incident on the photodiode 204, it is required that a thickness of the submount 210 (Hs) and a thickness of the Si substrate 211 (photodiode 204) (Hp) satisfy the condition Hs>Hp. Thus, accuracy in thickness is required. For that reason, manufacture is more difficult.